in the US for initially applying for the type O 90 day visa in the US, but for the one year extension it has to be in a Thai bank account, which is very easy to get on the 90 day visa, but almost impossible to get on any tourist visa, including the DTV, which is a tourist visa. There is also no limit on how many times you can get the type O 90 day visa.
The cheapest option all depends on how much money you have in the bank. As an example a multi-entry tourist visa costs US$200, where as a type O 90 day visa based on retirement costs bt2000 if you have the equivalent of bt800,000 in your personal bank account, and you can extend that for one year, every year, for bt1900.
You don't need an ED visa to actually go to school. You can learn Thai on the type B visa you need to get the work permit to run your business. You can not legally work on an ED visa. You'll also need at least bt2million in registered capital in the business to get your work permit, and up to four Thai employees, depending under what business structure it is set up under. At the very least you will need a licensed Thai accountant.
actually if you run out of money it is your responsibility to pay for your return to wherever you are going. If you overstay you are in jail until you can pay for your plane ticket, and fine.
What the government looks at now with reference to the bt500,000, and what it was designed for, to pay for any accidents, or health issues people with those DTVs, and if they are able to pay their bills. There's been some instances of people getting hurt, and not having the money they were supposed to have. When that gets abused too much then they change the rules.
you've lived here how long? The 180 day only makes you a tax resident. You are still on temporary stay terms technically no matter what visa you are on. You can be ideally correct, but realistically Thai considers you a resident of your passport country until you actually get citizenship.